There are many devices known in the prior art for supporting and translating a display web across a display screen or viewing surface. Such devices usually consist of a pair of reels on which the web is wound, and at least one controlled motor for driving the reels, often through a gear reduction train. In some systems one motor is used to drive each reel, so that the web direction may be reversed, either reel serving as a take-up reel.
In two motor systems, only one motor is actuated to drive the web at any given time, the other motor being idle. However, without the use of a clutch mechanism in the drive link between the motor and the reel, the idle motor will be turned by working motor through the translation of the web driving the supply reel and the associated gear train. Gear reduction systems generally are not adapted for such idling, and will deteriorate quickly under this sort of misuse. Clutch mechanisms, on the other hand, entail added cost as well as requiring frequent maintenance.
In display web systems it is often desirable or necessary to translate the web to a specific display position and stop the web exactly at that position. This proves to be a difficult achievement, due to the momentum of the motors, reels, and web. Often braking mechanisms are employed to counteract the momentum, although these mechanisms are costly. Alternatively, the motor is stopped with the web slightly short of the desired position, and the momentum is permitted to carry the web to the desired position. This method is approximate and often frustrating, as the momentum depends on a number of variable factors; i.e., the mass of the web on each reel, the speed of the reels and web at the instant of motor de-actuation, etc.